Generally, in a receiver receiving broadcast waves of AM broadcasting and FM broadcasting etc., a superheterodyne system is adopted as a receiving method. The superheterodyne system is a receiving system in which a received broadcast signal is mixed with a predetermined local oscillation signal so as to be converted to an intermediate frequency signal having a fixed frequency independent of the frequency of the reception signal (reception frequency), and thereafter subjected to a detection processing and amplification so as to be reproduced as a voice signal, and which is characterized in that the system is more excellent in sensibility and selectivity, etc. than the other receiving methods.
In the receiver adopting such superheterodyne system, a local oscillator, an antenna tuning circuit and a RF tuning circuit are all controlled by a same DC voltage (see, for example, pages 8 to 14, FIGS. 1 to 8 of Japanese Patent Laid-Open No. 4-358422). A resonance circuit is included in these circuits, of which resonance frequency can be changed by making variable the capacitance of a variable capacitance diode constituting a part of the resonance circuit. In this way, in the case where each resonance frequency is controlled by a common DC voltage, since the variable capacitance diode included in each resonance circuit has a similar temperature characteristic, the resonance frequency of each resonance circuit tends to vary in the same direction when the temperature of the receiver is changed, as a result of which a tracking error is arranged to be settled within a predetermined range without using a particular temperature compensating circuit.
In the conventional system in which each resonance circuit is controlled by a common DC voltage in this manner, since it is difficult to reduce the tracking error generated in both the lower limit frequency and the upper limit frequency of a reception frequency such as in the case where the reception frequency has a wide variable range, there is also provided a receiver in which each tuning frequency of the antenna tuning circuit and the RF tuning circuit is controlled by a DC voltage generated by a D/A converter independently of the common DC voltage (for example, see a column of embodiment of the above Japanese Patent Laid-Open No. 4-358422). Generally, when the DC voltage controlling the local oscillation is varied due to the temperature, the DC voltage outputted from the D/A converter is not varied in the same way as the DC voltage controlling the local oscillation, so that another temperature compensating circuit is needed. For example, a temperature compensating capacitor is used for apart of the antenna tuning circuit and the RF tuning circuit.
As another prior art for preventing expansion of the tracking error without using the temperature compensating capacitor in the receiver using the D/A converter, there is known a receiver in which the variation of the DC voltage is reflected on the D/A conversion output by using the DC voltage controlling the local oscillation as a reference voltage of the D/A converter (see Japanese Patent Laid-Open No. 2002-111527).
However, as disclosed in Japanese Patent Laid-Open No. 4-358422 described above, in the case where the tuning frequency of the antenna tuning circuit and the RF tuning circuit is controlled by using the D/A converter, although it is possible to prevent expansion of the tracking error by using the temperature compensating capacitor, there is a problem that the temperature compensating capacitor is generally expensive and component costs and the total cost of the receiver increase. For example, although a temperature compensating ceramic capacitor of which temperature coefficient can be selected from a plurality of temperature coefficients is commercially available, such capacitor is more expensive than a general purpose capacitor or other elements.
Since the above described temperature compensating capacitor is manufactured by a process different from a common semiconductor manufacturing process, there is also a problem that the capacitor can not be integrally formed on a semiconductor substrate with other components. Accordingly, even when components are made to be contained in a single chip, the temperature compensating capacitor needs to be attached as an external component, causing a cost increase due to an increase in man-hour for assembling.